Integral Logistics Management — Operations Management and Supply Chain Management Within and Across Companies

14.3.1 Operations-Oriented Finite Loading, or Operations Sequencing

Intended learning outcomes: Produce an overview on the technique (the algorithm) for operations-oriented finite loading, also called operations sequencing. Describe various priority rules.

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Operations-oriented finite loading aims to minimize possible delays to individual operations and thus the average potential delay of the entire production order.
Operations sequencing and operations-oriented finite loading are synonymous.

Overview: The individual operations are planned time period by time period on the basis of orders, starting from the start date determined by lead-time scheduling (Section 13.3.3).

Planning strategy: This means establishing meaningful rules of priority for the order in which operations are scheduled, with the aim of achieving maximum throughput. The queues waiting upstream of the work centers are monitored and adjusted.

Technique: The planning horizon is divided into time periods. The operations to be scheduled are then assigned to work centers, period by period, until the capacity limit is reached, regardless of the order to which they belong. Figure 14.3.1.1 demonstrates the principle of the resulting algorithm. This includes the following aspects:

Fig. 14.3.1.1       Technique (algorithm) for operations-oriented finite loading.

  • Work center priority: The order of the work centers becomes im­por­tant as soon as there is more than one operation to be scheduled for an order in each time period. Possibly, the subsequent operation then relates to a work center whose planning has already been carried out for this period and now must be revised.
  • Determine the operations to be scheduled in the first time period; typical operations are, firstly, every (subsequent) operation waiting for execution, for orders already started (the data on order progress identifies these operations), as well as, secondly, every first operation for orders not yet begun whose start date — calculated using a scheduling method (Section 13.3) — lies within the first time period.
  • Determine the operations to be scheduled in time period i, 2£ i£ n; candidates are, firstly, all operations not scheduled in the previous time periods; then, secondly, those operations for which the previous operation was scheduled in an earlier time period and whose start dates lie within time period i, as well as, thirdly, every first operation for orders not yet begun whose start date — calculated using a scheduling method (Section 13.3) — lies within the time period i.
  • Arrange the plannable operations by priority. The following secondary objectives may be applied to the selected order:
    A.   Minimize the number of delayed orders
    B.   Apply an equal delay to all orders
    C.   Minimize the average wait time for operations
    D.   Minimize the number of orders in process
  • The following priority rules may be applied (see also [RuTa85]):
    1.    The order in which the operations arrive (FIFO, “first in, first out”)
    2.    Shortest processing time rule (SPT)
    3.    Proximity of the order due date (EDD, earliest due date)
    4.    The ratio “remaining lead time for the order divided by the number of remaining operations”
    5.    The ratio “remaining lead time for the order divided by the time still available for the order” (SLK, shortest slack time rule, » order urgency; see also Section 13.3.6)
    6.    The ratio “remaining lead time for the order divided by the remaining operation time for the order”
    7.    (External) order priority
    8.    Any combination of the above

    Rules 1 and 2 are the easiest to apply in control of operations, be­cause the informa­tion is immediately available: it is physically visible “locally.” It is not necessary to consult a computer or a list. The other rules may require complicated calculations.

    Every priority rule takes into account one or another secondary objective. Rule 1 is often used, since it minimizes the wait time upstream of the work center and thus the average order delay (objectives A and B). If capacity is utilized more fully, the strategy changes, and rule 2 is chosen. This accelerates the largest possible number of orders and thus reduces the value of goods in process (objectives C and D).
  • Load the operations in order until the capacity limit is reached: If an operation exceeds the capacity limit, we transfer any as yet unscheduled operations to the next time period. The capacity used for the overlap load for the last operation is then no longer available in the next time period.

    One variation is not to schedule the operation that exceeds the capacity limit. However, this will use up remaining capacity only if an operation with a smaller load can be scheduled. This variation requires a more complicated algorithm.
  • Calculate the start date for the next operation: After loading the operation, we calculate its completion date and the start date of the next operation on the basis of the inter­operation time. To avoid problems with the algorithm (see “priority of the work centers” above), it may be useful to use the start of the next time period as the earliest start date.[note 1407]

Continuation in next subsection (14.3.1b).

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Course section 14.3: Subsections and their intended learning outcomes

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